The hardening, nitriding, nitrocarbiding, carbonizing, carbonitriding, and similar processes for metallic objects require that the workpiece be subjected to discrete periods of accurately controlled heating and/or cooling separated by periods during which the workpiece temperature is maintained steady, normally to equalize the temperature throughout the piece. To do this the workpiece is typically supported in a fluidized bed of heat-conducting particles so as to maximize heat exchange, as solid-solid heat exchange is highly efficient and allows accurate control of the process. A gas is fed through the bed at a volume/time rate sufficient to fluidize it and the bed is heated and/or cooled in accordance with the desired effect.
As mentioned above and described in "Warmebehandlung von metallischen Werkstoffen im Wirbelbettofen" (Sommer, Fachberichte Huttenpraxis Metallverarbeitung, volume 9/81) and "Einsatz von Wirbelschichten als Kuhlmedium" (also Sommer, ZwF 77, volume 9/82), the periods in which the workpiece temperature is radically changed are separated by periods during which the temperature remains relatively stable or becomes uniform throughout the workpiece. In other words the temperature/time curve of the bed has a first derivative which is considerable different from zero during the heating or cooling periods and which is about zero during the intervening steady-temperature periods.
The bed can be heated by the provision of coils or heating bars right in it for direct heating or outside the bed for indirect heating. Or it can be heated by a burner upstream and normally below the bed, or the burner can be effective against the sides of the vessel containing the bed. The bed is fluidized by a fan which forces enough gas through it to keep it fluid, and heat sensors are provided in or immediately downstream of the bed to ascertain the bed temperature. The heater or cooler is turned on high for the desired radical-heating or -cooling phases and is turned down for steady-temperature phases.
Such an arrangement can accurately heat treat a workpiece to produce a desired crystalline structure, but uses a great deal of energy. When the gas used to fluidize the bed is at a temperature that is quite a bit different from that the workpiece is to have, as in systems with a heating/cooling coil in the bed underneath the workpiece, the cooling or heating effect of this current of air must be compensated for by the heater or cooler. Even when the gas stream used for fluidizing is itself heated or cooled upstream of the bed so that it in turn heats the bed and the workpiece, it is necessary to heat or cool a much larger supply of gas than is actually necessary to heat or cool the workpiece. The gas exiting from the top of the bed during steady-state operation, for example, is virtually at the treatment temperature, representing a substantial expenditure of energy, and also requiring close monitoring and control of the process even during such steady-state operation.